This read-write value controls how the differentiated services codepoint (the high 6 bits of the IPv4 TOS byte or IPv6 Traffic Class) is set in this tunnel's outer IP header: -1 copies it from the payload's own header, -2 means a separate traffic conditioner determines it, and 0 through 63 sets it to that fixed value. Admins use it to control QoS marking behavior on tunneled traffic, either preserving the original packet's marking or imposing a specific one for the tunnel itself. For example, setting this to a specific DSCP value like 46 (expedited forwarding) ensures all traffic through this tunnel is marked for priority handling regardless of how the inner packets were originally marked.